Vacuum Fundamentals High-Vacuum Technology Course Week 3 Paul Nash HE Subject Leader (Engineering)
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Transcript of Vacuum Fundamentals High-Vacuum Technology Course Week 3 Paul Nash HE Subject Leader (Engineering)
Vacuum Fundamentals
• Final outstanding enrolments• Recap on last week
Measuring Vacuum
Gauge types, ranges and costs
Active and Passive Gauges
Location of gauges
Vacuum Technology
Vacuum Fundamentals
• To be able to describe a variety of vacuum measurement techniques
• To be able to select gauge types for a given range of vacuum
• To explain the difference between Active & Passive gauges
Learning Objectives
Vacuum Fundamentals
Vacuum GaugesThere are 3 phenomena used to measure
vacuum:
MechanicalDisplacement of materials
TransportMovement of gases
IonisationIon currents
Vacuum Fundamentals
Vacuum Gauges• Many different types of
gauges are available because each only covers a limited range of pressures
Range of gauge utility
Vacuum Fundamentals
Pirani gauge• The Pirani is a
dedicated low vacuum gauge device
• The resistance of the hot wire changes with the rate of heat loss (conduction) to the gas
• The Wheatstone bridge then measures the change in resistance of the hot wire
• Pirani’s are rugged and generally reliable and rarely need attention
Schematic Circuit for a Pirani (hot wire) gauge
Vacuum Fundamentals
Pirani calibration
• The calibration of a Pirani depends on thermal conductivity and so on the actual gas in the system
Correction Curve for Pirani Gauges
Vacuum Fundamentals
AdvantagesLow temp operating increases thermal stability by reducing thermal oxidisation of contaminantsSignificantly better resolution in the range above 100 TorrLong service lifeRobust and stable under normal working conditions
DisadvantagesCannot be used in high temp operations or located near heat sourcesResults can vary depending on the orientation of the gauge (preferred vertical)Can be easily contaminated particularly on installation
Pirani gauge
Vacuum Fundamentals
Penning (Cold cathode) Gauge
• A Penning gauge measures the ion current flowing from the cathode to the anode
• The magnetic field increases sensitivity by making the ions spiral as they travel to cause secondary ionization
Penning gauges require routine cleaning and
testing
Vacuum Fundamentals
Capacitance Manometer
• A = Annular electrode• D = Disk electrode• S = Substrate• G = Getter (in vacuum
space)• Differential capacitance
between annulus and disk depends on pressure difference between Test Chamber and “Getter”.
Vacuum Fundamentals
Ion gauges (Hot cathode)
•Pressures lower than 10-5 Torr can be measured with ion gauges
Vacuum Fundamentals
Interpreting the output
The gauge output must be interpreted and converted into a vacuum measurement
This output may also be used to control activation of:
Pumps
Other gauges
Valves and process control
Vacuum Fundamentals
Active GaugesMade possible by Surface Mount Technology on printed circuit boards
The gauge has an ID resistor to ‘tell’ the controller what type of gauge it is
Can also be interfaced to a PC Network for process control